1. Field of the Invention:
The invention is broadly related to cutting devices in the nature of shears and more particularly to mechanically actuated shears. More specifically, the invention is directed to mechanically actuated shears of the type in which the blades are generally flat or planar in nature and provide pairs of angularly related coacting edges which move relative to each other and cut with a scissors-like action. Shears of this type include cordless grass shears to which the invention is particularly directed.
2. Description of the Prior Art:
It is, of course, known to cut with both hand operated, e.g., scissors, pruning shears, and the like, and powered cutting devices, e.g., AC and battery powered grass shears. It is also known to shear with essentially parallel cutting edges moving relative to each other, e.g., as in a powered sheet metal shear. In contrast, hand as well as powered scissors, grass shears, hedge clippers, and the like, perform the cutting action with what may be termed a scissors type action. The scissors type cutting action can be characterized by observing that it involves pairs of blades whose coacting cutting edges perform their cutting action by moving the edges together in some angular relation such that the point of blade edge intersection, i.e., the "cutting point" , moves outwardly. This action can be observed in an ordinary pair of hand scissors in which the pivoted blades ordinarily wipe against each other at the cutting point. A type of scissors-like action can also be observed in a conventional hedge trimmer where the triangular shaped teeth of one blade move with respect to the teeth of the other blade and between each pair of coacting cutting edges the cutting point moves outwardly at some speed as the edges move through a cutting action.
The subject of improving the shape and nature of cutting edges which cut in a scissors-like action have been the subject of prior patents both with regard to mechanically actuated as well as hand actuated cutting devices. See, for example, U.S. Pat. No. 636,473 directed to a blade edge construction for a pair of hand operated scissors and U.S. Pat. No. 3,316,636 directed to a mechanically actuated pair of blades which operate in a more linear rather than a scissors-like action. One can observe the shapes of pruning shears, pinking shears, surgical shears, and other manually operated scissors-like cutting devices to realize that blade shape significantly affects both the ability to grasp as well as the ability to cut particular materials. It can also be observed from the numerous prior art cutting devices which operate in a scissors-like manner that blade shapes which lend themselves to hand operated cutting operations do not necessarily lend themselves to cutting when the same blade shapes are mechanically actuated and particularly at relatively high speeds. Also, different materials lend themselves to being cut at different speeds as illustrated by use of variable speed drills.
Various electrical powered shears have long been known such as electric hedge clippers, and the like. In an early U.S. Pat. No. 432,433 there is a teaching of an electric vibrator type battery powered shear. As to shear blade construction, the use of a fixed blade having a set of teeth in conjunction with a movable blade having a set of mating teeth such that grass is caught and sheared between the tooth edges is also old in the art. This general blade construction is found in mowers for cutting hay and tall grass as well as in hedge trimmers. However, considerable power is required to move such blades and the blade fingers are generally rigid, inflexible and heavy in weight. Such characteristics are required when cutting hay, tall heavy grass, and the like. The blade edges have generally been formed as straight edges and this practice has existed for many years. Since large amounts of power have been available, e.g., gasoline engines, AC motors, there has been no particular concern for refinements in blade efficiency, blade power, speed of cutting, or the like. However, the modern suburban home has many small areas and narrow edges of substantially light grass around trees, flower gardens, shrubs, sidewalks, animal fencing, adjacent steps, and the like. In this environment, the source of power is limited since many of these areas and edges are remote from electrical receptacles and the prior art, until recently, did not provide a dependable, lightweight, cordless type shear specifically for cutting small light grass areas and edges to keep the same trim and neat. Consequently, refinements in conventional cutting practices become more important.
While apparently not previously recognized, a characteristic obtained in a mechanically driven reel type lawn mower having a helical and flat blade is that the cutting point moves at a uniform speed while the helical blade is driven at a uniform speed.
To meet the needs of the modern suburban home, an improved lightweight portable battery powered grass shear was introduced into the market in recent years and is best described in U.S. Pat. No. 3,623,223. The type of cordless shear described in this patent has proved to be extremely popular particularly for cutting small narrow width, grass areas and edges associated with suburban homes as previously described. As best explained in the patent, this type of shear employs as blades a pair of thin metal plates at one end of which the cutting teeth are formed. By fixing the bottom blade and using a battery powered motor to reciprocate the top blade on a pivotal mount, the individual teeth of the top blade wipe the teeth of the bottom blade and a scissors-type cutting action is obtained. The teeth have been formed with straight cutting edges and it can be observed in this type of shear that the reciprocated top blade teeth in their to and fro movement move from zero speed to a maximum speed and then back to a zero speed in each cutting cycle. Thus, it can also be observed that the cutting edges of the top blade teeth are moving at a non-uniform speed and that the power requirements for driving the reciprocated top blade vary substantially during a cutting cycle. It also means, as the present invention observes, that when the coacting edges of the respective teeth are formed as straight lines and the top blade is driven at a non-uniform speed that the contacting cutting point between each pair of contacting edges on the top and bottom blade teeth moves at a non-uniform speed. Thus, the power required to cut the same material at one cutting point is or may be substantially different from the power required to cut at another cutting point.
Stated in a more general context, the prior art has heretofore failed to recognize the significance of blade shape on cutting in a scissors-like action when the blades are mechanically actuated in a manner to cause the contacting cutting point between each pair of coacting edges to move at a widely varying speed rather than at a uniform speed.